In the chemical pulping of cellulosic fibrous material, after the active cooking chemicals are consumed, the cooking liquor is typically recycled to recover as much reusable chemicals and energy as possible. For example, in both continuous and batch kraft digesters, spent cooking liquor, that is, black liquor, is typically removed from the digester and directed to evaporators to increase the solids concentration of the liquor and recover water. The concentrated liquor is then fired in a recovery boiler to remove the remaining moisture and produce sodium salts from which cooking liquor, that is, white or green liquor, can be produced to be used for digestion.
Instead of sending the used liquor to recovery, the hot, pressurized black liquor removed from the digester may be used to treat chips prior to the formal cooking process. For example, as disclosed in copending applications "Continuous Kraft Cooking with Black Liquor Treatment", Ser. No. 08/299,103, filed Sep. 2, 1994 now abandoned and "Kraft Cooking with Spent Liquor Pretreatment of Chips", Ser. No. 08/345,822, filed Nov. 21, 1994 now abandoned. Hot black liquor may be recirculated sequentially to pretreat wood chips during the impregnation stage. In a batch process, black liquor removed from the digesters may be stored at different temperatures in black liquor "accumulators" for reuse in pretreating chips.
Also, the hot, pressurized black liquor may be allowed to expand under controlled conditions, i.e., "flashed", to produce a source of steam and to reduce the moisture content of the liquor. The steam so produced can be used elsewhere in the pulping process. For example, flashed steam can be used directly to presteam chips prior to cooking.
The above flashing process, though it has been successfully employed in conventional continuous digesters, has the drawback that the steam produced contains volatile compounds, including sulfur compounds, which are undesirable in the presteaming of wood chips. Typically, wood chips are steamed at atmospheric pressure, or slightly above, such that the residual gases not absorbed by the wood chips must be collected and treated. Treatment typically is by combustion in a mill's noncondensible gas (NCG) system. However, this collection and treatment system becomes particularly significant when the steam used contains volatile compounds, including sulfur compounds, which have undesirable environmental impact, including noxious odor. It is therefore preferable to use a source of steam which minimizes or eliminates the introduction of volatile compounds to the steaming process.
Also, the flashing of kraft black liquor in flash tanks causes the formation of foam. The large size of conventional flash tanks is primarily due to the volume of foam that is produced and must be accommodated.
The present invention as applied to continuous or batch digesters, reduces the size and number of flash tanks and minimizes the introduction of volatile compounds to the chip steaming process. To achieve this, the invention includes a cooking liquor recovery system in which the heat energy of the extracted cooking liquor is recovered and steam produced by means of indirect heat exchangers, and one or more smaller flash tanks, or reboilers. In such reboilers, the hot, pressurized spent cooking liquor is passed in heat exchange relationship with volatile-compound-free, "clean" water to heat the water above its boiling point, or flash point. The hot water can be subsequently flashed to clean steam in one or more flash tanks to produce one or more sources of clean steam. The steam produced may be at one or more pressures and temperatures.
The system according to the invention, including the flash tanks thereof, can be at least 50% smaller than conventional steam recovery systems and may be located at ground level, whereas conventional, cascading flash tanks must be elevated.
The clean steam produced according to the invention can be used where needed in the pulp mill. For example, the clean steam can be used for presteaming wood chips. When used to treat chips, since the steam does not introduce volatile compounds to the presteaming process, the load of volatile compounds which must be collected and treated by a mill's NCG system is reduced.
The steam may also be used for indirect preheating of cooking liquors or filtrates that are used in and around a digester, for example, kraft white liquor, black liquor and washer filtrate, or sulfite or soda cooking liquors. The clean condensate produced from indirect heating of liquors can be used as needed in the mill, for example, as washer shower water or combined with other condensates.
Furthermore, since clean water does not foam, the flash tanks may be much smaller, at least approximately 50% smaller, than conventional black liquor flash tanks.
The present invention provides for a heat recovery and steam generation system which is much smaller than conventional systems and produces a clean source of steam for chip treatment and other uses.
According to one aspect of the present invention a method of producing steam having less noncondensible gases than steam produced by conventional black liquor flashing is provided. The method comprises the steps of: (a) passing hot black liquor at a temperature of about 120-165.degree. C. from a digester through a heat exchanger; and (b) passing an evaporable liquid to be evaporated through the heat exchanger into heat exchange contact with the hot black liquor so that the evaporable liquid is evaporated to produce steam having less noncondensible gases than steam produced by conventional black liquor flashing.
Step (b) may be practiced by heating clean water as the evaporable liquid in the heat exchanger, and then flashing the heated clean water into clean steam in at least one flash tank (preferably two or more different flash tanks to produce steam at two or more different pressures), which are smaller than conventional and may be at ground level. The clean steam may be used in a bleach plant for bleaching chemical cellulose pulp, or otherwise directly brought into contact with comminuted cellulosic fiber material that is used to produce chemical cellulose pulp, such as in a chip bin, or brought into contact with a chemical cellulose pulp itself.
The heat exchanger may comprise an evaporator, such as a falling film evaporator (although other types may also be utilized), and there may be the further step of passing the black liquor through an inductor to increase the velocity thereof (preferably at least about 20%) prior to passing the black liquor into the heat exchanger (evaporator). Again, desirably clean water is passed into the heat exchanger as the evaporable liquid so that step (b) produces clean steam, and there may be the further step of recirculating some of the black liquor which is passed through the heat exchanger to the inlet to the heat exchanger to thereby reduce the temperature of the entering black liquor for more efficient heat transfer (since there is a lower temperature difference between the heat exchanging fluids). Alternatively, step (b) may be practiced by introducing some of the black liquor that is passed through the heat exchanger as the evaporable liquid (e.g. recirculating between 10-40% of the black liquor after it passes through the heat exchanger as the evaporable liquid). In this case the steam produced is not "clean" but it does have less NCG than conventional steam produced by flashing black liquor in flash tanks.
Steps (a) and (b) may be practiced utilizing at least one boiler (e.g. two boilers) as the heat exchanger, and step (b) is preferably practiced using clean water as the evaporable liquid to produce clean steam. There may also be the further step (c) of cooling the black liquor at least 5.degree. C. (e.g. from about 105-110.degree. C. to about 95-100.degree. C.), and preferably about 10.degree. C., after it passes through the at least one boiler by passing it into heat exchange relationship with the liquid. Preferably step (c) is practiced by using green liquor or white liquor as the liquid for cooling, although water may also be used where hot process water is needed at some point in the pulp mill.
According to another aspect of the present invention a steam producing system for a cellulose pulp is provided. The system comprises in the following elements: A digester having a black liquor outlet. A heat exchange means having a hot liquid inlet, a hot liquid outlet, an evaporable liquid inlet, and an evaporable fluid outlet. The black liquor outlet connected to the hot liquid inlet. And a source of evaporable liquid connected to the evaporable liquid inlet.
The heat exchange means may comprise a liquid-to-liquid heat exchanger, an evaporator, such as a falling film evaporator (although other types are also utilizable), one or more boilers, or other structures that are capable of ultimately producing steam. The source of evaporable liquid preferably comprises clean water. The system--especially where liquid-to-liquid heat exchangers are utilized--preferably includes at least one flash tank substantially at ground level connected to the evaporable fluid outlet for flashing the evaporable liquid into steam (after the evaporable liquid has been heated by the heat exchanger).
The system may also further comprise an inductor connected between the black liquor outlet and the heat exchange means, particularly where the heat exchange means comprises an evaporator. The inductor increases the speed of the black liquor (e.g. at least about 20%). The system may also further comprise means for recirculating about 10-40% of the black liquor from the hot liquid outlet to the hot liquid inlet (again to make the heat transfer more efficient). Where the heat exchange means comprises at least one boiler, there typically is further a cooler connected to the hot liquid outlet, and to which green liquor, white liquor, or water to be heated passes to cool the black liquor at least about 5.degree. C. while simultaneously heating the cooling liquid.
Since the invention recovers energy from liquor by means of liquid-to-liquid heat exchangers, it is also more energy efficient than conventional flashing systems. By transferring heat from a hot liquor stream to a cooler liquid stream, a higher level of energy (temperature) can be transferred to the cooler liquid and then used where needed. The flashing of liquor as is conventional limits the heat recovered to the temperature of the flashed steam. That temperature is dependent upon the pressure present during flashing and is lower than the original liquor temperature. The invention is not so limited and thus has improved heat transferability compared to the conventional methods.
Further, the invention can recover more turpentine than is recovered with conventional flashing systems. Turpentine is conventionally recovered from either digester relief steam or flashed liquor steam. However, when flashed black liquor steam is used to preheat (that is presteam) wood chips, the turpentine (or its source material, terpenes) when reintroduced to the cooking process can be destroyed. By not reintroducing terpene-laden steam to the cooking process, more turpentine can be recovered.
According to this aspect of the present invention, a method of recovering maximum heat energy from hot spent digester cooking liquor is provided. The method comprises the steps of: (a) Passing hot spent digester cooking liquor in liquid-to-liquid heat exchange relationship with a clean evaporable liquid so that the temperature of the clean evaporable liquid is raised above its evaporation temperature. (b) Expanding the heated clean evaporable liquid under controlled conditions to produce a clean source of steam. And (c) treating comminuted cellulosic fibrous material with the clean steam from step (b) prior to introduction into a digester from which the hot spent digester cooking liquor is withdrawn as the spent liquor used in step (a). The clean evaporable liquid is typically water, but in some situations it can instead be a cooking or wash filtrate liquid. In that case the method comprises the steps of: (a) Passing hot spent digester cooking liquor in liquid-to-liquid heat exchange relationship with a liquor that can be used to treat comminuted cellulosic fibrous material in a digester so that the temperature of the liquid is raised; and (b) using the heated liquid in the digester to cook the comminuted cellulosic fibrous material. The liquid is heated preferably comprises cooking liquor or wash filtrate, the cooking liquor may be kraft white liquor, black liquor, green liquor, sulfite cooking liquor, or soda cooking liquor.
It is the primary object of the present invention to provide an improved method and system for producing cleaner steam, preferably clean steam from black liquor, to thereby achieve numerous advantages including reducing the load of volatile compounds which must be collected and treated in a pulp mill NCG system, improving heat recovery, and improving turpentine recovery. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.